Devices of the genus-forming type are applied as central locking systems in motor vehicles. An example is described in DE 44 30 315 C2 as the prior art. Despite the alternating code values, misuse is possible. Such misuse can, for example, be carried out when there is an authorized attempt to unlock the central locking system. In this case, the message which is emitted is received by a person attempting illegal intervention and recorded. The reception of the message at the same time is disrupted by a radio frequency signal which lies within the reception bandwidth of the central locking receiver, but not in the reception bandwidth of the receiver of the person attempting illegitimate intervention. In this way, the person attempting illegal intervention can record a message and transmit it again which is detected as valid in the receiver.
In order to avoid such illegal interventions, a transmitter and receiver with a clock may be provided to compare the transmission time of the respective message with its reception time, as disclosed for example in DE 44 30 315 C2. A transmission with divergent timing, and thus an illegal intervention or an unauthorized attempt at unlocking can thus be detected in the receiver. However, this device has the disadvantage that a battery provided in the transmitter is additionally discharged by the clock, which is disadvantageous in particular when the transmitters are miniaturized—for example when embodied as a chip card.